Metro spoke with the TTC to break down the breakdown.

Thousands of Torontonians were over 40 minutes late on Thursday morning as the TTC experienced major delays on the Yonge-University-Spadina subway line. Subway service inched to a crawl and passengers triggered the emergency system 14 times. Mayor John Tory and TTC spokesperson Brad Ross apologized for the delays. Metro spoke with the TTC to break down the breakdown.

1. The TTC recently consolidated contractors, and a former contractor took their equipment from a service room at St. Patrick Station when they left. But they mistakenly took components needed to make track signalling work at that station.

2. With track signalling down at St. Patrick, the 1950s-era system defaulted to its fail-safe to prevent subway cars from travelling through, because the system could no longer detect if there was a safe distance between each vehicle.

3. To overcome the fail-safe, TTC crews had to manually override the system for each set of subway cars at St. Patrick. This caused the frustrating sense of transit inching forward in bumper-to-bumper traffic.

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4. While the signal failure happened at St. Patrick, the damage spread throughout Line 1 in a domino effect that lasted almost eight hours. Subway trains backed up behind St. Patrick Station and stations around St. Patrick accumulated huge crowds of passengers because trains weren't arriving as frequently.

5. Automatic train control – the modern update to the 1950s track signalling system – will be fully ready by the end of 2019 and it will increase capacity on Line 1 by 25 per cent by managing shorter gaps in each subway train. Let's just hope no one swaps out the wrong equipment.